The family Equidae originated in North America. Species of the genus Equus,
which includes living horses and zebras, first appeared in the early Pliocene,
around five million years ago. Thereafter they differentiated into dozens of
species, many of which lived in the New World (including South America) until
the end of the Pleistocene. All had the same basic body plan, reflecting adaptations
for swift running. Although in Europe Equus was pictured frequently
by Stone Age artists, direct association between humans and horses in the American
Pleistocene has not been confirmed. The half-dozen or so species of New World
Equus disappeared at or before 11,000 years ago. However, well prior
to its disappearance in the New World, Equus managed to invade Eurasia
by crossing the Bering landbridge westward. In Eurasia, one species, E.
caballus, was eventually brought into domestication; around AD 1500, this
species was reintroduced into the ancestral homeland of Equus by the
Spanish conquistadores.

See
also: Kurtén, B., and E. Anderson, 1980. Pleistocene Mammals
of North America. Columbia University Press: New York.